Polyolefin Separator

Polymer Composition

Manufacturing these membranes involves the use of polyethylene or polypropylene to create a thin and flexible sheet. Polyethylene is favored for its lower melting point which enables an early thermal shutdown, while polypropylene offers better mechanical strength and chemical resistance. Many modern cells use a tri-layer structure that combines both materials to gain the benefits of each.

The choice of polymer affects the wettability of the separator, which dictates how quickly the electrolyte can be absorbed during the filling process. These materials must remain stable over thousands of cycles without degrading in the presence of highly reactive lithium salts.

Thermal Shutdown

Safety features are often built directly into the porosity of the polyolefin separator to prevent thermal runaway. When the internal temperature of the cell reaches the melting point of the polymer, the microscopic pores collapse and stop the flow of ions. This shutdown mechanism effectively kills the electrochemical reaction and prevents further heating.

If the temperature continues to rise, the membrane must remain physically intact to prevent a hard short circuit between the electrodes. Ceramics are sometimes coated onto the polyolefin to increase this melt-down temperature and provide an extra layer of protection.

Porosity Control

Performance of the battery is closely linked to the size and distribution of the pores within the membrane. High porosity allows for faster ion transport and better high-power performance but can weaken the mechanical strength of the film. Designers must balance the need for low resistance with the requirement that the separator must resist penetration by lithium dendrites.

The thickness of the polyolefin separator has been steadily reduced to increase the energy density of the cells, but this trend increases the difficulty of manufacturing. Consistent pore geometry is necessary for ensuring a uniform current distribution across the entire electrode surface. The quality of this membrane is a primary factor in the overall reliability of the battery.